Enterprise Navigation

Historical NC Landslide Events

Landslide Events in North Carolina

Some examples of landslide events in North Carolina are presented here. Scroll down for photographs, diagrams, and descriptions.

Landslide presentation

The following images were included in a MS PowerPoint presentation used by North Carolina Geological Survey geologists from the Asheville Regional Office at many public landslide outreach meetings. The presentation has been adapted to the Internet for broader distribution. This page is on "Rock Slope Stability." Links to other topics appear in the contents shown above.

Slide numbers correspond to those of the original MS PowerPoint presentation. Slide numbers "missing" are slides that were turned into text. Captions are from the original presentation.

Slide 30 - Landslides and landslide related fatalities from the mid-July 1916 hurricane in Transylvania County, NC. Damage from landslides and flooding occurred over much of the south-central mountain area. The July 15-16, 1916 flood is considered the flood of record in western North Carolina.

Slide 31 - Color-infrared aerial photograph of the Gorges State Park area, Transylvania County, North Carolina showing mapped locations of deposits left by the catastrophic failure of Lake Toxaway Dam on August 13, 1916. The dam failure triggered a debris flow along the Toxaway River that traveled over 7 miles and into South Carolina. Lake Jocassee is underlain by cobble, gravel and sand deposits from the flood. The original dam was in about the same location as the current dam. The quote shown is from S.W. McCallie, State Geologist of Georgia at the time. Studies by the North Carolina Geological Survey estimate that the outflow just below the dam was on the order of 293,938 cfs (discharge) and 50 mi/hr (velocity). Information from Geology of Gorges State Park, N.C. Geological Survey Information Circular 31.

Slide 32 - Top: View looking downstream along the Toxaway River below the dam showing the assumed scour lines and the location of cross section D (bottom) used to reconstruct the super elevation angle of the dam failure torrent. This information goes into computing an estimated velocity and discharge of the outflow.

Slide 33 - Top Left: 60-foot long boulder weighing nearly 900 tons transported by the flood waters along the Toxaway River from the August 13, 1916 Lake Toxaway Dam failure. Photograph taken about 0.5 miles downstream from Toxaway Falls. Top Right. Imbricated boulders at the crest of the boulder levee shown in red in the cross section at Bottom Left. Bottom Right. Photograph of the contact (shown by arrow) of the boulder flood deposits overlying pre-existing flood plain deposits along the Toxaway River.

Slide 34 - Left: Detailed map of a ~4 acre active weathered-rock slide along the Toxaway River in Gorges State Park. This slow-moving landslide was probably triggered by the 1916 dam failure torrent that eroded and over-steepened the slope along the river. Right: Tree ring studies of trees on and off the slide indicate a period of slide movement during the 1965-1974 timeframe corresponding to a period of above average rainfall. Tree ring studies were done cooperatively with the U.S.F.S. Coweeta Hydrologic Laboratory, Otto, N.C. Information contained in Geology of Gorges State Park, N.C. Geological Survey Information Circular 31.

Slide 35 - Trees growing on an active landslide are commonly curved. The tree tilts with the moving slide, and over time attempts to regain vertical growth resulting in the curved trunk. A and B. Curved trees showing the effects of movement on the Toxaway River weathered-rock slide. White arrow points to person for scale in photo A (left).

Bottom Right. Debris flows and debris slides (shown in red) triggered by the Aug. 10-17, 1940 hurricane. Base map is a georegistered Sept. 1940 aerial photograph of the Blue Ridge Escarpment area near Deep Gap in Watauga County (unregistered photograph courtesy of U.S.G.S.). Light colors in the main stream and river channels are sediment from the debris flows and flooding. The flooding and landslides from this event killed 26 people in North Carolina alone. At least two fatalities resulted from a landslide along the Watauga River.

Slide 39 - Boulders line the surface of a small debris fan deposit in Watauga County believed to be deposited by a debris flow triggered by one of the the August 1940 storms. Afor sale sign is posted on the tree for this property.

Slide 40 -Upper Right: Light Detecting and Ranging (LiDAR) hillshade derived from elevation data from the North Carolina floodplain mapping program greatly aids in landslide hazard mapping. Hillshade image of the Seven Devils-Foscoe area along the Watauga River southwest of Boone showing some debris fans and colluvial deposits (indicated by red arrows).

Slide 41 - The November 2-6, 1977 tropical depression triggered flooding and landslides across western North Carolina. The map shows the location of numerous debris flows triggered by this storm (red) in the Bent Creek watershed near Asheville.

Slide 42 - Map and charts showing location, velocity, rainfall and soil data for the Lands Creek Debris Flow I. The velocity of a debris flow can be estimated by determining the banking, or super elevation, angle it makes as it rounds channel bends, the radius of curvature of the channel, and the channel gradient. A velocity of 23 mi/hr is about 33 ft/sec. Debris flows are particularly dangerous because they often happen without warning and move very rapidly downslope. The destroyed mobile home and chlorinator were built on pre-existing debris fan and flood deposits.

Bottom Right: Mud line in a tree left by the debris flow next to new foundation pads for the chlorinator building for the Bryson City municipal water system. The debris flow destroyed the original chlorinator building.

Slide 45 - Rainfall vs. elevation chart for rain gauge stations in the region of the Lands Creek Debris Flow I. The elevation of the initiation point for the Lands Creek debris flow is approximately 3000 ft. Rainfall amounts are generally greater at higher elevations for a given storm event. Rainfall data courtesy of U.S.F.S. Coweeta Hydrologic Laboratory and WHBN in Bryson City.

Slide 46 - Rainfall intensity as much as rainfall amount is as important in triggering debris flows. Graph shows plots of 1 hr and 24 hr rainfall intensity readings from the U.S.F.S. Coweeta Hydrologic Laboratory for the Nov. 2-6, 1977 storms and the Dec. 23, 1990 storm that triggered the Land Creek I debris flow. The graph shows relative rainfall intensity and return periods for the two events. Note that for the Dec. 1990 event the rainfall intensity and return period is greater for the higher elevation rainfall gage (Coweeta-31).

Slide 47 - Views of the Lands Creek Debris Flow II track and debris (left side of both images) that went into the Bryson City reservoir. Reservoir was no longer a water supply source at the time of the debris flow, and has since been drained (lower photo). This debris flow originated from the same road as Lands Creek Debris Flow I.

Slide 48 - Hydrologic and rainfall data, radar map, and map showing the locations of the Charley Branch debris flows triggered by the 3-day rain event in Swain County in May 2003 that caused extensive flooding and landslides. Rainfall and streamflow data from U.S.G.S. Radar image from National Weather Service.

Slide 49 -Top: Cross section through the path of the the Charley Branch 5 debris flow track showing the debris flow superelevation angle and cross sectional area used to compute the estimates of velocity and discharge.

Slide 51 - This is a portion of the geologic map of southwestern North Carolina (NCGS, 1992) near Bryson City in Swain County. The red dots show locations of slope failures that occurred with summer thunderstorms in May of 2003. From this perspective it is hard to see any correlation between the slope failures that coincide with areas underlain by sulfidic-graphitic bedrock. Studies so far indicate that steep slopes underlain by sulfidic-graphitic rock are more susceptible to landslides.

Right: Schematic block diagram showing typical debris flow track and deposit. Over thousands of years debris fans accumulate at the toes of slopes from multiple debris flow and flood deposits. Debris fans are attractive building sites because of moderate slopes above the main flood plains, and the general lack of bedrock excavation required for roads and foundations. Renewed debris flow activity originating in the source area can put developments on debris fans at risk.

Slide 57 - Steep, high excavations in debris fan deposits can be unstable. This cut slope failed during the May 5-7, 2003 rains in Swain County. Although the log cabin remained intact, the failed debris pushed it 3-5 feet off its foundation.

Slide 58 - Demolished remains of a residence at the location of a fatal debris flow on Dec. 11, 2003 near Maggie Valley. Much of the demolition took place during the effort to rescue the victim buried in the back of the house. The embankment failure that originated in the scarp in the background mobilized into a debris flow. A broken water supply line, the road embankment, and a buried dark line marking the location the original ground surface can be seen in the scarp. A lawsuit pending against the Maggie Valley Sanitary District and the N.C. Department of Transportation claims that the leaking waterline caused the embankment failure. At this time it is not known if the water line was leaking, and if it was, what caused the leak.